This invention relates to a novel curable resin composition useful particularly as a paint, which contains as a main component an epoxy group-containing polymer or compound which can be cured by crosslinking reaction.
Epoxy group-containing polymers or compounds react with crosslinking agents and are crosslinked and cured, and the resultant curing products have excellent performance, and therefore, are widely used in fields such as paints, adhesives and sealers. As the crosslinking agents can be used amines, polyaminoamides, acid anhydrides, phenol resins, urea resins, melamine resins, isocyanate compounds, blocked polyisocyanate compounds, etc. having functional group(s) capable of reacting with epoxy group(s).
However, at the time of reaction of the epoxy group-containing polymers or compounds with these crosslinking agents, the epoxy groups are ring-opened to form secondary hydroxyl groups; and the secondary hydroxyl groups remain as such and therefore become a cause to lower the water resistance and weather resistance of the curing products. Furthermore, these crosslinking agents, sometimes, react gradually with the epoxy group-containing polymers or compounds even at room temperature, and are thickened and gel, and thus curable resin compounds comprising both these components are insufficient in storage stability.
Crosslinkable polymer compositions in which epoxy group-containing polymers, polyfunctional active ester compounds and reaction accelerators are compounded have hitherto been proposed as those obviating these drawbacks (see Japanese Patent Publication No. 34370/1993). In the gazette, as the polyfunctional active ester compounds are exemplified phenyl esters, thiophenyl esters, halogenated phenyl esters, nitrophenyl esters, alkyl-substituted phenyl esters and naphthyl esters of aliphatic polyvalent carboxylic acids or aromatic polyvalent carboxylic acids; benzoate esters, acetate esters and propionate esters of polyhydric phenols; etc.
The present inventors have examined the crosslinkable polymer compositions, and as a result have found that in curing reaction of these compositions, there is no formation of secondary hydroxyl groups by ring opening of the epoxy groups, but in the case of the epoxy group-containing polymers and polyfunctional active ester compounds only, they are not sufficiently crosslinked and cured unless they are heated about 160.degree. C., and in order to crosslink them at temperatures lower than the temperature, it is necessary to compound reaction accelerators such as trialkylamines and salts of crown ethers in large amounts, and as a result disadvantages arise that the electrical insulation properties of the compositions themselves are lowered, and there arise dangers at the time of electrostatic coating.
Further, as to the above crosslinkable polymer compositions, the compatibility of the polyfunctional active ester compounds, which are crosslinking agents, with organic solvents is not sufficient, and large amounts of the solvents are necessitated to give uniform solutions, and therefore, it is difficult to make the solutions high solids, and moreover use of large amounts of the solvents is also undesirable in view of the saving of resources and prevention of environmental pollution, etc.
The present inventors have intensely studied for the purpose of obviating the above-mentioned drawbacks of such a crosslinkable polymer composition as stated above which comprises epoxy group-containing polymers, polyfunctional active ester compounds and reaction accelerators, and with a view to developing a curable resin composition which has good storage stability at a room temperature, in which a crosslinking reaction rapidly takes place even at a temperature as low as about 100.degree.-120.degree. C. without any use of reaction accelerator in a large amount, and which has also good electrostatic coating properties. As a result, they have found that the purpose can be accomplished by, in place of use of the above active ester compounds as crosslinking agents, making functional group(s) in each of which a carbonate ester group (--O--CO--O--) is directly bound to an aromatic ring, coexist in the same molecule as the epoxy group-containing polymer.
It has further been found that when a compound having in one molecule at least two functional groups in each of which a carbonate ester group (--O--CO--O--) is directly bound to an aromatic ring, is used as a crosslinking agent for an epoxy group-containing compound, the crosslinking agent is not thickened nor gels in a mixed system with the epoxy group-containing compound during storage; and moreover the compound is excellent in solubility in organic solvents, it is possible to make the solutions high solids, and free secondary hydroxyl groups, etc. do not remain in the curing product.